Vacuum arc remelting and electroslag refining are well known for the production of high quality steels. Though both processes are in widespread use today, neither process approaches an ultimate ideal in the sense of producing steel having an optimum combination of cleanliness and structure.
The ESR process, for example, decreases sulphur to very low levels but it cannot produce the exceedingly low H, O, and N levels which are demanded by certain high quality applications today, such as lens quality molding practices. The ESR slags are very sensitive to moisture which can, and often does, have a significant impact on H values. Indeed, variability sufficient to impact on final gas values can occur between slag batches and suppliers. Even weather can be a factor. When a heat is run in a hot, humid mid-summer day as contrasted to a cool, dry fall or winter day, the final H content may be, and often is, higher during the humid operating conditions. Very low sulphur values are characteristic of the ESR process but it is impossible to avoid some contamination in the form of inclusions since the process is one in which a liquid slag is in continuous contact with the liquid steel and thus there is always the possibility of undesired migration of inclusions from the slag to the metal at the slag-metal interface.
Certain other drawbacks which are a function of the inherent operation of the ESR process have also been noted. For example, the possibility of coarse dendritic structures being formed in the ESR process is always present, these structures being attributable to the rather sharp "V" shape of the molten pool which in turn adversely affects the solidification pattern with a consequent increase in the formation of dendritic structures.
The vacuum arc degassing process, as exemplified by U.S. Pat. No. 3,501,289, on the other hand can yield very low H, O, and N levels. Low sulphur levels can also be attained, but this must be achieved primarily by careful control of the desulphurizing additions prior to the vacuum electric arc and violent stirring phase. Nonetheless, qualities which approach aircraft quality specifications can be consistently attained using the vacuum arc degassing process. However, it has been determined that for some very critical applications, such as lens quality mold steel, the vacuum arc degassing process cannot alone provide the consistently proper structure which is available by vacuum arc remelting.
By lens quality mold steel is meant typical mold or stainless steel intended to take and maintain a high polish and which is used, for example, in such applications as extrusion dies or molds for clear glass or plastic parts. Examples of such parts are the clear plastic or glass shields covering the instrument panel in automobiles, the clear shield which forms part of aircraft fighter pilot headgear or TV tubes.
Stated generally, many mold steel applications require superior homogeneous steels which can maintain a high hardness level at operating temperatures while providing lens quality polishability free of defects that may impart flaws to the finished part. These steels must also be capable of resisting fatigue cracking, heat checking, thermocycling, impact loading failures, and must have isotropic properties. Today the material of choice is AISI H-13 steel which is a 0.4C-5Cr-1.5Mo-1V hot work tool steel which is used for extrusion, forging and die casting applications. This material however does not always produce the desired characteristics when produced by methods currently used in the trade.
Thus there exists a need for a means of producing high quality premium steels intended for demanding and/or critical applications such as extrusion, die casting, or forging dies.